This invention relates to a method of growing crystals of .beta.--BaB.sub.2 O.sub.4 from a fluxed melt. More particularly, the invention relates to the growth of such crystals from a melt comprising barium borate, sodium oxide and sodium chloride, in certain proportions.
Beta-barium borate, the low temperature form of BaB.sub.2 O.sub.4, is an important non-linear optical material which has been shown to be useful for second harmonic generation in laser systems. [D. Eimerl, L. Davis, S. Velsko, E.K. Graham, and A. Zalkin, J. Appl. Phys. 62(5), 1968-1983 (1987)]. More particularly, the use of such a material is, for example, in the environment of lasers as a "doubler" to obtain a different frequency light from a laser having a fundamental frequency. The material is particularly useful because it has been shown to be an efficient harmonic generator, has a high optical damage threshold, and is essentially transparent to ultraviolet light as short as 190nm in wavelength.
Barium borate typically melts congruently at 1095.degree. C., but the solid that crystallizes just below this melting point is a centrosymmetric crystal which does not exhibit non-linear optical properties and is not useful in second harmonic generation. Thus, it becomes important to appreciate that below a temperature of about 925.degree. C., barium borate exists in another crystal form, usually called the beta-barium borate form.
This low temperature form was originally thought to be C2/c, but current work has found this form to be R3c. This beta form can be crystallized from solution below this phase transition temperature, i.e., below about 925.degree. C. An essential requirement is that a solution is required which will dissolve BaB.sub.2 O.sub.4 below this transition temperature and will then supersaturate the .beta.--BaB.sub.2 O.sub.4 upon cooling to cause crystals of .beta.--BaB.sub.2 O.sub.4 to nucleate and grow, or to cause a dipped "seed" crystal to grow in size.
A number of important criteria must be met for selection of the solvent components of such a solution. First, the solvent atoms should be either components of the crystal itself, or they should be atoms which do not enter the crystal lattice as impurities to such an extent as to degrade the desired crystal properties. Secondly, the solvent components should be compatible with the crucible and furnace materials at the growth temperature. Thirdly, the solution should have a sufficient range of supersaturation to inhibit the nucleation of crystals in cooler parts of the crucible away from the seed crystal. Finally, the solution should be compatible with a crystal growth rate slow enough to yield high quality crystals, but not sufficiently slow as to make it commercially undesirable.
In the past, crystals of .beta.--BaB.sub.2 O.sub.4 have been grown at the Fujian Institute in the People's Republic of China from solutions of BaB.sub.2 O.sub.4 with BaCl.sub.2, BaF.sub.2, Li.sub.2 O.sub.3, Na.sub.2 O, B.sub.2 O.sub.3 and Na.sub.2 B.sub.2 O.sub.4 (J. Aidong C. Fen L. Qi, C. Zusheng, and Z. Yong, J. Crystal Growth 79, 963-969 (1986)). The best crystals by admission were obtained from solutions with Na.sub.2 O. However, the actual growth of the crystal was too slow to be commercially acceptable.
Barium borate crystals were first synthesized in 1874 by melting together NaBO.sub.2 and NaCl, followed by cooling. From the resultant solid, the barium borate was obtained in the form of small, needle-form crystals on removal of the NaCl from the product by soaking in water. (R. Benedikt, Ber. Deut. Chem. Ges. 7, 703 (1874)).
Thus, up to now pure .beta.--BaB.sub.2 O.sub.4 crystals have not been grown at a commercially acceptable rate, and those crystals which are of acceptable quality for optical applications take so long to grow that other less expensive alternatives must be sought.
The present invention, avoids these disadvantages and provides a method which permits growth of good optical quality .beta.--BaB.sub.2 O.sub.4 crystals at acceptable rates.